In the formulation of coatings, it is well-known that rheological modifiers may be added to control the flow properties of the final product for a particular application. A large number of rheological additives are available to control properties of a coating formulation during storage, transportation, processing, application, and post-application to a particular surface. These additives include modified and unmodified organic clays, and a large variety of organic compounds and inorganic compounds such as silicas, as set forth, for example, in Rheology Handbook, Rheox, Inc., Hightstown, N.J. (1991 ).
The control of sag, levelling, and pigment suspension in a coating formulation requires additives which maintain the proper viscosity of the coating at a variety of shear rates. This is difficult, since control of these properties involves opposing phenomena. For example, total freedom from sag requires a coating with extremely high viscosities just after application; however, the low flow rate of such a material means that the coating may not flow out completely to form a smooth film. The development of an enveloping rheological structure to control pigment suspension during storage may result in coating which exhibits levelling problems during application.
To maintain viscosity under these diverse conditions, the prior art has suggested incorporation of additives, such as surfactants and thickeners, to control pigment suspension along with additional additives, such as organic clays, intended to control levelling and sag. However, production of a coating with such multiple additives is a difficult and complex procedure and sometimes requires separate processing conditions for incorporation of each rheological additive. A correct balance of properties must also be achieved among the various additives. In the prior art, even processing parameters of single rheological additives have proven quite difficult to control. If these precise formulation requirements are not employed, coatings with inconsistent properties may be produced. These precise formulations are also difficult to manufacture on a large scale, and require inventorying of a large variety of additive products.